Projection objective for immersion lithography

• US 20050225737A1
• Filed: 12/20/2004
• Published: 10/13/2005
• Est. Priority Date: 12/19/2003
• Status: Active Grant

• Current Assignee: CARL Zeiss SMT GmbH (Carl-Zeiss-Stiftung)
• Original Assignee: Carl Zeiss AG (Carl-Zeiss-Stiftung)
• Inventors: Pazidis, Alexandra, Gruner, Toralf, Brotsack, Markus, Schuster, Karl-Heinz, Weissenrieder, Karl-Stefan, Zaczek, Christoph, Loering, Ulrich, Hirnet, Alexander, Schink, Harald, Scheible, Patrick, Lill, Michael, Scheible, Guenter
• Granted Patent: US 7,460,206 B2
• Time in Patent Office: Days
• US Class Current: 355/53
• CPC Class Codes: G03F 7/70341 Details of immersion lithography aspects, e.g. exposure media or control of immersion liquid supply chemical composition of immersion liquids G03F7/2041
• CPC Class Codes: G03F 7/70983 Optical system protection, e.g. pellicles or removable covers for protection of mask

Abstract

In a projection objective provided for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium arranged between a last optical element of the projection objective in the light path and the image plane, the last optical element has a transparent substrate and a protective layer system that is fitted to the substrate, is provided for contact with the immersion medium and serves for increasing the resistance of the last optical element to degradation caused by the immersion medium.

Claims

1. A projection objective for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium arranged between an optical element of the projection objective and the image plane,

the optical element having a transparent substrate and a protective layer system that is fitted to the substrate, is provided for contact with the immersion medium and serves for increasing the resistance of the optical element to degradation caused by the immersion medium.

2. The projection objective as claimed in claim 1, wherein the optical element is the last optical element of the projection objective in the light path.

3. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that is essentially impermeable to the immersion medium.

4. The projection objective as claimed in claim 3, wherein the barrier layer comprises at least one barrier layer material that is essentially chemically resistant to the immersion medium, and is essentially free of pores passing through from an outer side of the barrier layer that is remote from the substrate to a side of the barrier layer that faces the substrate.

5. The projection objective as claimed in claim 3, wherein the barrier layer is a single layer.

6. The projection objective as claimed in claim 3, wherein the barrier layer is formed as a multilayer layer.

7. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that contains at least one of the following fluoride materials or essentially consists of such a material:

actinium fluoride (AcF3), bismuth fluoride (BiF3), erbium fluoride (ErF3), europium fluoride (EuF3), gadolinium fluoride (GdF3), holmium fluoride (HoF3), potassium magnesium fluoride (KMgF3), lanthanum fluoride (LaF3), sodium yttrium fluoride (NaYF4), neodymium fluoride (NdF3), samarium fluoride (SmF3), terbium fluoride (TbF3), titanium fluoride (TiF3), thulium fluoride (TmF3), vanadium fluoride (VF3), ytterbium fluoride (YbF3), yttrium fluoride (YF3).

8. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that contains at least one of the following oxide materials or essentially consists of one of said materials:

silicon dioxide (SiO2), magnesium aluminum oxide (MgAl2O4), aluminum oxide (Al2O3), tungsten dioxide (WO2), tungsten trioxide (WO3).

9. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer made of an oxidic material having a high packing density, the packing density preferably being more than 95%, in particular more than 97% or 98%, of the density of the bulk material and/or an average refractive index of the oxidic material deviating less than 5%, preferably less than 3%, in particular less than 2%, from the refractive index of the bulk material.

10. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that essentially consists of an ion-sputtered oxide material, in particular silicon dioxide.

11. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that essentially consists of an oxide material applied in a PECVD method, in particular PECVD silicon dioxide.

12. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that has an optical layer thickness of between 0.15 λ

and 0.6 λ

, in particular between approximately 0.2 λ and

0.3 λ

, or between approximately 0.4 λ and

0.6 λ

, where λ

is the operating wavelength of the projection objective.

13. The projection objective as claimed in claim 12, wherein, for a refractive index difference Δ

n between the refractive indices of the barrier layer material and of the immersion medium, Δ

n>

0.04 holds true.

14. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that is designed as an antireflection layer.

15. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer, and an optical layer thickness of the barrier layer is adapted to the optical properties of a monolayer or multilayer dielectric layer system adjoining the barrier layer in such a way that a reflection-reducing effect results in conjunction with the layer system.

16. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer that is applied directly to an exit-side surface of the substrate.

17. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer, an antireflection layer system being arranged between the substrate and the barrier layer.

18. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer, an antireflection layer system being applied to a surface of the barrier layer that is remote from the substrate.

19. The projection objective as claimed in claim 17, wherein the antireflection layer system is a magnesium fluoride/lanthanum fluoride alternating layer system.

20. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one barrier layer made of an organic material that is essentially impermeable to the immersion medium, in particular made of a perfluorinated fluorocarbon, preferably polytetrafluoroethylene.

21. The projection objective as claimed in claim 1, wherein the protective layer system is designed as a graded index layer with a continuous or discontinuous refractive index profile perpendicular to a layer extent, a refractive index in a region near the substrate essentially corresponding to the refractive index of the substrate material and a refractive index in a region provided for contact with the immersion medium essentially corresponding to the refractive index of the immersion medium.

22. The projection objective as claimed in claim 1, wherein the protective layer system is designed as a wear system that is optimized in terms of refractive index in such a way that a gradual material dissolution caused by contact with the immersion medium does not lead to a substantial change in the optical properties of the protective layer system.

23. The projection objective as claimed in claim 1, wherein the protective layer system comprises a bulk material plate that is provided for contact with the immersion medium and is made of a plate material whose refractive index is in proximity to the refractive index n_I of the immersion medium, a refractive index difference Δ

n with respect to the immersion medium preferably being less than 0.01, in particular less than 0.005.

24. The projection objective as claimed in claim 23, wherein the immersion medium essentially consists of water and the plate material is lithium fluoride (LiF), or wherein the immersion material is a fluorinated liquid and the plate material is sodium fluoride (NaF) or calcium fluoride (CaF₂) or lithium fluoride (LiF).

25. The projection objective as claimed in claim 23, wherein the bulk material plate is exchangeable (exchangeable plate).

26. The projection objective as claimed in claim 1, wherein the protective layer system has an effective refractive index n_ss at least in a region adjoining the immersion medium, such that Δ

n<

0.05, preferably Δ

n<

0.01, in particular Δ

n<

0.005, holds true for a refractive index difference Δ

n=|n_I−

n_ss| with respect to the refractive index n_I of the immersion medium.

27. The projection objective as claimed in claim 1, wherein at least one dielectric antireflection layer system having one or a plurality of single layers is arranged between the substrate and the wear system.

28. The projection objective as claimed in claim 1, wherein the protective layer system consists of, at least in a region adjoining the immersion medium, a mixed material having at least one material having a low refractive index and at least one material having a high refractive index, in which case, preferably, the material having a low refractive index has a refractive index n_L<

{square root}{square root over (n_I·

n_S)} and the material having a high refractive index has a refractive index n_H>

{square root}{square root over (n_I·

n_S)}, where n_I is the refractive index of the immersion medium and n_S is the refractive index of the substrate material, and a ratio of the material having a low refractive index and the material having a high refractive index being chosen such that an average refractive index n_MIX of the mixed material is present.

29. The projection objective as claimed in claim 28, wherein the average refractive index n_MIX is set such that it is in proximity to the refractive index n_I of the immersion medium, in which case Δ

n=|n_I−

n_MIX|<

0.05 preferably holds true, in particular Δ

n<

0.01 holds true.

30. The projection objective as claimed in claim 28, wherein the average refractive index n_MIX≈

{square root}{square root over (n_I·

n_S)}.

31. The projection objective as claimed in claim 1, wherein the protective layer system consists of a mixed material is a monolayer antireflection layer having an average refractive index n_MIX≈

{square root}{square root over (n_I·

n_S)}±

2% and having an optical layer thickness of approximately I·

d_QWOT, where d_QWOT is the layer thickness of a quarter wave layer of the mixed material and I is an odd integer.

32. The projection objective as claimed in claim 28, wherein the mixed material is constructed as a nanopatterned multilayer material.

33. The projection objective as claimed in claim 28, wherein the mixed material has a continuous mixture of two or more components.

34. The projection objective as claimed in claim 1, wherein the substrate consists of a fluoride crystal material, in particular calcium fluoride.

35. The projection objective as claimed in claim 1, wherein the optical element provided with the protective layer system, in particular the last optical element, is a planoconvex lens having a spherically or aspherically curved entry face and an essentially planar exit face to which the protective layer system is fitted.

36. The projection objective as claimed in claim 1, wherein the optical element provided with the protective layer system is an essentially plane-parallel plate.

37. The projection objective as claimed in claim 36, wherein the plane-parallel plate is wrung onto an optical element or is connected thereto optically neutrally in a different way.

38. The projection objective as claimed in claim 1, wherein the optical element provided with the protective layer system is exchangeable.

39. The projection objective as claimed in claim 1, wherein the protective layer system comprises at least one layer that is made of an immersion liquid and is arranged within the protective layer system (double immersion).

40. The projection objective as claimed in claim 1, wherein the protective layer system is essentially fitted only to an image-side exit face of the optical element provided with the protective layer system.

41. The projection objective as claimed in claim 1, wherein the protective layer system is fitted to an image-side exit face of the substrate and also extends continuously over adjoining side areas of the substrate.

42. The projection objective as claimed in claim 1, wherein the protective layer system essentially covers all outer areas of the substrate.

43. The projection objective as claimed in claim 1, wherein the protective layer system comprises a multilayer system having a first layer with first defects and at least one second layer with second defects, the first defects and the second defects being distributed over different lateral positions in the layers in such a way that a defect-free region exists in at least one of the layers essentially at every location of the multilayer system.

44. The projection objective as claimed in claim 43, wherein an interface between the first layer, which is nearer to the substrate, and the second layer, which is further from the substrate, is configured as a polishing interface produced by polishing the first layer prior to coating with the second layer.

45. The projection objective as claimed in claim 1, wherein the protective layer system is formed by an essentially plane-parallel protective plate that is wrung onto a substrate surface of the substrate.

46. The projection objective as claimed in claim 45, wherein the substrate consists of a fluoride crystal material, in particular of calcium fluoride, the protective plate consisting of synthetic quartz glass.

47. The projection objective as claimed in claim 45, wherein the substrate consists of calcium fluoride and the protective plate consists of barium fluoride.

48. The projection objective as claimed in claim 45, wherein the protective plate has a plate thickness of less than 5 mm, the plate thickness preferably being less than 2 mm and more than 50 μ

m.

49. The projection objective as claimed in claim 1, wherein the projection objective is assigned a temperature regulation device for temperature regulation of the optical element provided with the protective layer system.

50. The projection objective as claimed in claim 49, wherein the temperature regulation device is formed as a cooling device for cooling the optical element provided with the protective layer system to a temperature below an ambient temperature of the projection objective.

51. The projection objective as claimed in claim 1, wherein the projection objective has an image-side numerical aperture NA≧

0.80, preferably NA≧

0.98, in particular NA≧

1.

54. The method as claimed in claim 52, wherein the protective layer system is designed as recited in claim 1.

63. The optical element as claimed in claim 58, wherein the protective layer system is designed as recited in claim 1.

52. A method for protecting a projection objective, which is designed for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium arranged between an optical element of the projection objective and the image plane, against degradation of optical properties caused by the immersion medium, comprising:

fitting of a protective layer system provided for contact with the immersion medium at least to an exit side of the substrate.

53. The method as claimed in claim 52, wherein the protective layer system is fitted to the last optical element of the projection objective in the light path.

55. The method as claimed in claim 52, wherein the fitting of the protective layer system comprises the following:

production of a first layer of a multilayer system on the substrate or on a coating applied on the substrate;

removal of a portion of the first layer by polishing in order to produce a polishing surface;

production of a second layer on the polishing surface of the first layer.

56. The method as claimed in claim 55, wherein the following are carried out at least once:

removal of a part of the second layer by polishing in order to produce a polishing surface of the second layer;

production of a third layer on the polishing surface of the second layer.

57. The method as claimed in claim 55, wherein, between the removal of a part of a layer and the production of a further layer on the removed layer, a cleaning is carried out for the removed layer.

58. An optical element having:

a transparent substrate; and

at least one protective layer system that is fitted to the substrate, is provided for contact with an immersion medium and serves for increasing the resistance of the optical element to degradation caused by the immersion medium.

59. The optical element as claimed in claim 58, wherein the substrate consists of a fluoride crystal material, in particular calcium fluoride.

60. The optical element as claimed in claim 58, which is designed as a planoconvex lens having a spherically or aspherically curved first face and an essentially planar second face to which the protective layer system is fitted.

61. The optical element as claimed in claim 58, wherein the optical element is an essentially plane-parallel plate.

62. The optical element as claimed in claim 58, wherein the protective layer system is fitted to a first outer area of the substrate and also extends continuously over adjoining side areas of the substrate, the protective layer system preferably essentially covering all outer areas of the substrate.

66. The optical system as claimed in claim 64, wherein the optical element is designed as recited in claim 58.

64. An optical system having:

at least one optical element provided for contact with an immersion medium;

the optical element having a transparent substrate and a protective layer system that is fitted to the substrate, is provided for contact with the immersion medium and serves for increasing the resistance of the optical element to degradation caused by the immersion medium.

65. The optical system as claimed in claim 64, which contains, in addition to the optical element provided for contact with the immersion medium, at least one further element that is preferably not provided for contact with an immersion medium.

67. A method for protecting a projection objective, which is designed for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium arranged between an optical element of the projection objective and the image plane, against degradation of optical properties caused by the immersion medium, comprising:

use of a cooled immersion medium with a media temperature lying below an ambient temperature of the projection objective.

68. The method as claimed in claim 67, wherein the media temperature is set to less than 15°

C., the media temperature preferably being set to a temperature in the range of between 10°

C. and 5°

C.

69. The method as claimed in claim 67, wherein an immersion medium which essentially consists of water is used.